When am I suppose to sign the contract for my job in the Air Force?

Alright just started MEPS in March and finish in May where i was put in DEP. Originally i was suppose to start on October next year but received a call from my recruiter saying that a job is available for me and that i will leave on the 21st next week. Do to lack of communication, I was unable to get a hold of my recruiter as well for showing up at the office when he not there so I'm trying to see if anyone can provide info of when i need to sign the contract to get the job. I leaving to BMT on the 21st however I'm just making sure that I still have time to sign it.